The present invention relates to a system for controlling valve timing for intake and exhaust valves of an automotive engine, and more particularly to a valve timing control system having a hydraulic valve lifter.
It has been proposed to provide a valve timing control system for the automotive engine, which operates to reduce duration of valve opening at low speed in a light load range in order to reduce fuel consumption and to stabilize engine operation. At high speed in a heavy load range, the duration of the valve opening is increased in order to produce a large engine power. There have been proposed various methods for controlling the valve timing. If a lift displacement of the valve is changed, the duration is changed. For example, when the lift displacement is increased, the duration is increased. Accordingly, a method is proposed to provide a hydraulic valve lifter between a cam and a valve stem and to provide a control system for controlling pressure of oil in the valve lifter.
Japanese Patent Application Laid-Open 61-93216 discloses a valve timing control system in which a hydraulic valve lifter has a cup-shaped outer lifter, a hollow inner lifter engaged with the outer lifter, and a high pressure oil chamber provided between the outer and inner lifters. A check valve is provided in a passage communicated with the oil chamber. The check valve is operated by an electromagnetic valve to be opened for controlling the pressure of oil in the oil chamber.
In the system, pressure receiving area of the outer lifter engaging the valve is equal to pressure receiving area of the inner lifter engaging the cam. Consequently, an amount of the valve lift is equal to or smaller than that of the cam lift. Therefore, it is difficult to increase the duration of valve opening and to reduce a size of a cam mechanism.